Conventionally, in the processes of producing ordinary rubber pneumatic tires (hereafter simply referred to as tires), tires having been finally completed through a vulcanization process are subjected to an appearance inspection and a uniformity test, and tires having passed these inspections are shipped. In this kind of uniformity test, for example, a tire to be tested is mounted on the rim body of the mounting rim of a tester, air is filled into the tire at a predetermined pressure from a gas filling portion installed in the rim body, and a characteristic measurement test is carried out.
For example, a tire uniformity tester is used for this characteristic measurement test. In the characteristic measurement test, in a state in which a predetermined load is applied to a tire mounted on a rim so that the tire is pressed against a rotation drum, the distance between a spindle and a drum shaft is fixed and the tire is rotated, force variations generated in the tire are measured. These force variations are mainly three variations, that is, a force variation in the radial direction of the tire (radial force variation, hereafter referred to as RFV), a force variation in the lateral direction of the tire (lateral force variation, hereafter referred to as LFV) and a force variation in the tangential direction of the tire (tractive force variation, hereafter referred to as TFV).
For the purpose of measuring these force variations, a method in which the drum shaft is driven by driving the spindle or a method in which the spindle is driven by driving the drum shaft is used usually as a driving method. Furthermore, as the measurement locations of the force variations, there is a case in which the measurement is carried out at the drum shaft and a case in which the measurement is carried out at the spindle. The force variations generated in the tire are mainly caused by non-uniformity in hardness and thickness of rubber and depending on circularity. The measured load is subjected to frequency analysis and tracking analysis, and only the rotation speed components and harmonic components of the tire are extracted.
The load to be applied to the tire is several hundred kgf in the case of the tire of a passenger car. The rotation speed of the tire is specified at 60 rpm in the case of low-speed uniformity (JIS-D4233, 2001), and the RFV and the LFV are mainly measured.
In a general measuring apparatus, the entire performance of the measuring apparatus is checked with reference to a test piece having known values (for example, the prototype kilogram in the case that the general measuring apparatus is a weight measuring apparatus); however, such checking is impossible in the case of this kind of tire uniformity tester. The reason for this is that there is no tire whose force variations are completely known.
This is because the characteristics of the tire changes due to degradation caused by the effect of temperature and repeated use. Hence, although the accuracy of an arithmetic unit inside the measuring apparatus can be checked by inputting an electrical simulation signal, even if an attempt is made to examine measurement errors due to mechanical factors, since reference input itself does not exist, it is very difficult to inspect the accuracy of the entire apparatus.
Accordingly, as disclosed in Patent Reference 1, the accuracy of the entire system of the tire uniformity tester is inspected by using a statistical method based on numerous tire data obtained by carrying out sequentially repeated measurements an appropriate number of times and by confirming whether the range of data variation is limited to a predetermined value or less. Conventionally, on the basis of totally 100 pieces of data obtained by subjecting 10 tires to sequentially repeated measurements carried out 10 times, a dispersion analysis in a two-way layout with two factors, i.e., a factor among tires and a factor among the number of measurement times, is carried out to confirm that the standard deviation values of residual errors (experimental errors and measurement errors) are within specified ranges. This kind of test is referred to as a 10×10 test (ten by ten test).